Thrust force calculation in axial bellow modelling

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s muthuraj
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Joined: Wed 25. Sep 2013, 11:37

Thrust force calculation in axial bellow modelling

Post by s muthuraj » Mon 28. Oct 2013, 07:18

Dear sir

I am new to Rohr2..I have a question regarding axial bellow modelling.
In rohr2 there are three options(entire force,bellow and no force) in thrust force calculations.
In a header calculation the anchor loads on both sides of axial bellow are very high if i use entire force option but it is very very less if i go for bellow option.
For example i am getting 101kN anchor loads with entire force option but with bellow option it is just 10kN.
The support design will also become critical for 101KN load.
The stresses are also failing in some cases if i go for entire force option but that is not the case with bellow option.
Kindly rely for this query.

Thanks in advance

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Posts: 422
Joined: Wed 14. Sep 2011, 08:23

Re: Thrust force calculation in axial bellow modelling

Post by rohr2support » Wed 6. Nov 2013, 13:14

Hi Rohr2 User

If you use axial or universal expansion joints these are usually made of bellows without tie-rods.
This means that the axial force due to internal pressure that usually is present in all segments of a piping system cannot be retained by tie-rods and the bellow is not designed to take it.
Therefore you need to design the piping and the supports so that these forces can be taken care of.

Here is an example of a piping system with axial expansion bellows and internal pressure:
The internal pressure is acting on the internal cross-section of the pipe but also on the bellow waves:
In normal situations you should include the full internal pressure force (from internal cross section and bellow) in the calculation.
In some cases, like in this situation where the bellow is directly on a pump nozzle:
the forces on the pump nozzle are only determined by the bellow forces.
Therefore you may include only bellow forces at the expansion joint at the left side but in order to corectly calculate the loads on the pump anchor, you need to add the
missing force from internal pressure on the internal cross section of the pipe onto the pump housing.

In ROHR2 you also have the options not to consider the axial pressure force at all, but this generally is a bad idea.
As you have seen in your example, the axial pressure forces get very high quickly with pressure and surface area.

Best regards

Rohr2 Support
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Joined: Wed 20. Dec 2017, 08:07

Re: Thrust force calculation in axial bellow modelling

Post by arshia43 » Fri 2. Feb 2018, 07:29

Axial expansion joints with rod does not have possibility to move in axial direction and they can just move in lateral and angular direction. So may be it should be ok to model interal pressure(axial force) on then with "no force"option of the rohr2. Is it accptble from your point of view?

Best regards

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Re: Thrust force calculation in axial bellow modelling

Post by rohr2support » Mon 5. Feb 2018, 09:34

If you have rods with your bellows, this means that the axial direction of your expansion bellow is stiff.
You can still modell them as "axial expansion joints" but you should set the axial stiffness to a very high value.
If you do so, it doesn't make any difference if you include the internal pressure reaction force or not (as it will end up in the stiff axial spring rate of the bellow).

Keeping the flexible axial spring rate of the bellow without rods and set the reaction force to "none" usually is not correct.

Best regards

Rohr2 Support

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